238 PHOTOSYNTHESIS 



On account of the many precautions which must be taken and the time 

 consumed in the various steps of the process it is too complex and tedious 

 for making a large number of determinations. 



The absorption of oxygen takes place much more slowly and finally 

 ceases when considerable quantities of oxide are formed. It is possible 

 to dissolve this coating of oxide in a solution of ammonium carbonate. 

 Oxygen is rapidly and completely absorbed when brought in contact with 

 copper and a solution of ammonium carbonate ; however, in this process 

 considerable carbon dioxide is given off. This latter difficulty can be 

 avoided by the use of metallic copper in contact with solution ( containing 

 undissolved pieces) of ammonium sesquicarbonate in a saturated solu- 

 tion of ammonium hydroxide specific gravity 0.90. Such a combination 

 has an absorption capacity about 24 times its volume of oxygen. It is 

 essential that only bright copper be used. The combination can be used 

 in a pipette as designed for phosphorus. It has a distinct advantage over 

 phosphorus of absorbing oxygen completely even at — 7°, and has also 

 a much higher absorption coefficient than potassium pyrogallate. The 

 mixture also absorbs carbon monoxide and acetylene so that it cannot 

 be used for analytical purposes in the presence of these gases. In gas 

 mixtures containing more than 30 per cent oxygen the absorption is not 

 quantitative. The tension of ammonia in this solution is not great but 

 for accurate analytical work it is advisable to wash the residual gas 

 with water or dilute sulfuric acid before making readings. 



A higher absorption, fifty to sixty times its volume of oxygen, can 

 be obtained by using a solution made by saturating with ammonium chlo- 

 ride a mixture of one part concentrated ammonia and one part of water. 2* 



Sodium hyposulfite is an excellent absorbent of oxygen : NaoSoOi + 

 HoO + O — '^ 2NaHS03. Durig has used the following solution with 

 success: 50 grams of NasSoO* are dissolved in 250 cc. of water and 

 before using mixed, excluding air, with a solution of 30 grams NaOH 

 in 50 cc. of water. The absorption pipette must be protected against 

 the air by a bulb containing water and mineral oil. On account of the 

 corrosive action of this reagent on glass a special technique is necessary 

 which requires some skill ; also it is advisable to coat the inside of the 

 absorption pipette with a thin covering of gutta percha. The reagent 

 has an extremely high absorption coefficient, about 40 air analyses can 

 be made with 100 cc. solution. It is advisable to pass the gas mixture 

 into the absorption pipette at least twice. 



An improvement in the sodium hyposulfite method has been devised 

 by Fieser^^ by the use of anthraquinone-p-sulfonate. For use in a 

 shaking pipette the following solution is recommended : 16 grams NasSoO* 

 (86.7 per cent), 6.6 grams NaOH (sticks), 2 grams anthraquinone-(5- 



Kraut's "Handbuch der Anorganischen Chemie," 7th Ed.. Vol. S, Part I, pp. 7.36, 

 741 1564. It is advisable therefore to pass the gas finally over iron filings. 



^Badger, W. L., Jour. Ind. Eng. Chcm., 12, 161 (1920). 



'^Fieser, Jour. Amer. Chem. Soc, 46, 2639 (1924). 



